Do you have the ability to put in VST after the VSTi? If so, there are several plugins that will turn a stereo signal into a mono signal. Here's one, Moneo, on this page:

http://www.betabugsaudio.com/plugs.php

Here's another location with the same Moneo plugin and a couple others:

http://www.untidymusic.com/wordpress/vst...ur-mix-in-mono/

Now, with all of that said, I would still provide the engineer with individual instrument stereo tracks, if that is what they are natively, and let him/her figure out what really needs to be mono.

-Scott